Add new system url request context getter to the blimp engine.

blimp/engine/app/blimp_url_request_context_getter.h

Index: blimp/engine/app/blimp_url_request_context_getter.h
diff --git a/blimp/engine/app/blimp_url_request_context_getter.h b/blimp/engine/app/blimp_url_request_context_getter.h
index 43d5395b43912e9304ab45d8cef2d68b87d9e2b9..a991d6e07b47c482e1716619de11eef4b34aa949 100644
--- a/blimp/engine/app/blimp_url_request_context_getter.h
+++ b/blimp/engine/app/blimp_url_request_context_getter.h
@@ -29,6 +29,8 @@ class ProxyService;
 namespace blimp {
 namespace engine {
 
+// The URLRequestContextGetter for Blimp "user" requests.
+// System request context is handled in a separate class.
 class BlimpURLRequestContextGetter : public net::URLRequestContextGetter {
  public:
   // The content of |protocol_handlers| is is swapped into the new instance.